It is well known how to attempt to protect the jet of the cast metal againt oxidation by the surrounding air when casting metals using an open jet. Among the numerous methods for protecting jets of cast metal used up to the present time, mention can be made of the process for casting metal in molds described in French Pat. No. 2,165,769, by virtue of which, on the one hand, the surface of the molten metal is made inert when it is heated up by injecting an inert gas through an injection rod passing through the top of the lid of the heating ladle containing the said metal and, on the other hand, previous inert gas injection into the mold into which the metal is to be cast. This process has the disadvantage that there is a risk of oxidation when the mold is transferred via the ambient air from its inert gas injection station to its casting station, and also during the casting operation as such.
This problem of protecting the casting jet occurs particularly acutely when casting non-ferrous metals which can form solid metal scums at the impact point of the jet cast metal, and more particularly when casting zinc in ingot molds.
It is generally a well-known fact that at any point where liquid zinc falls freely by gravity in a jet, and in particular when casting zinc in ingot molds, a solid metal scum forms at the point of impact of the jet of cast metal and then extends over the surface of the liquid contained in the ingot mold.
As is well known, the formation of this scum results from the oxidation of zinc. Bubbles are formed at the point of impact of a casting jet because of the high surface tension of zinc (.perspectiveto.780 dynes/cm). The air carried along with the jet brings about rapid surface oxidation by these bubbles. Since zinc oxide (melting point of ZnO.perspectiveto.1950.degree. C.) is solid at the casting temperature (470.degree. C. to 520.degree. C.) and, in addition, its specific heat is substantially higher than that of zinc (ZnO: Cp=12 cal.mole.sup.-1 T.sup.-1 ; Zn:Cp=7.5 cal.mole.sup.-1 T.sup.-1), the bubbles are immediately stabilized and solidified. The solid metal scum thus formed appears as a very fine foliated structure which contains about 98% metal and represents 1 to 1.5% of the cast metal.
In order to avoid faults caused by the oxides coming from this scum in the end-product zinc ingots, it is necessary to remove the surface layer of scum from the surface of the molten metal by manual skimming, generally with a metal scraper before the metal in the ingot mold is completely solid. Thus, in addition to the drawback of a not-inconsiderable loss of the original metal, the formation of zinc scum involves an investment in manpower and time which adversely affects the profitability of the whole process.
It is well known that, in order to avoid the formation of zinc scum, casting can be carried out in differently designed plants, such as plants using rising pouring techniques. But, as is also known, rising pouring is more costly and more difficult to operate than gravity casting.